The invention relates to a method of and a device for determining the position of a medical instrument, introduced into the body of a patient, relative to a periodically moving organ of the body.
Clinical applications impose increasingly more severe requirements on the precision of the determination of the position of a medical instrument introduced into the body of a patient. Notably the growing interest in minimal-invasive methods for the treatment of cardiac diseases necessitates the development of methods and devices allowing the physician to guide a medical instrument to an accurately predetermined position inside or outside the heart. For example, in the case of direct myocardial revascularization it is necessary to guide a catheter with a small drill head to a plurality of predetermined positions on the ventricle wall in order to drill small holes therein or to administer a medicine directly to the ventricle wall in such positions.
According to a method which is known from Gepstein et al. xe2x80x9cA Novel Method for Non-Fluoroscopic Catheter-Based Electroanatomical Mapping of the Heartxe2x80x9d, Circulation 1997; 95:1611-1622, the position of a catheter introduced into the body is measured in that on the catheter there is mounted an electromagnetic transmission device, for example, an RF coil, whereas a corresponding electromagnetic receiving device, for example a plurality of RF receiving coils, is arranged outside the body in order to receive the signals transmitted by the transmission device. Even though the position of the catheter, or the tip of the catheter, relative to the coordinate system of the receiving device, i.e. the position of the catheter in space, can thus be comparatively accurately determined, the position of the catheter relative to the surrounding anatomy, for example relative to the heart, cannot be determined in this manner. To this end, additionally X-ray fluoroscopy images would have to be formed during the treatment; such images would enable tracking of the catheter in continuously formed new X-ray images. Such fluoroscopic methods, however, are comparatively intricate on the one hand and do not provide the accuracy required for the determination of the position of the medical instrument. On the other hand, the continuous formation of X-ray images during the treatment of the patient represents an additional X-ray load. There is another problem in that the anatomy surrounding the medical instrument is not stationary during the treatment but moves, notably with a periodic motion. This holds above all for the heart which performs a periodic eigenmotion, i.e. which contracts during the systole and expands during the diastole and is also subject to an additional, practically periodic motion which is due to the respiration of the patient. Be it partly to a lesser extent, this holds not only for the heart, but also for other organs such as the brain, the stomach and the liver which are also moved by the cardiac and respiratory motions.
Therefore, it is an object of the invention to provide a method and a device which enable as accurate as possible determination of the position, relative to a periodically moving body organ, of a medical instrument introduced into the body of a patient.
This object is achieved by means of a method as disclosed in claim 1 and a device as disclosed in claim 10.
The invention is based on the recognition of the fact that the periodic motion of the body organ with respect to which the position of the medical instrument is to be determined can also be taken into account for the position determination. To this end, a periodic motion signal which is associated with the periodic motion of the body organ is measured, for example a respiratory motion signal which is dependent on the respiration of the patient or an electrocardiogram which is associated with the cardiac motion, while the spatial position of the medical instrument and of a reference probe is determined by means of a measuring device, for example by means of the known electromagnetic measuring device. The reference probe is then arranged outside the body of the patient, for example on the surface thereof or on the patient table, and is constructed in such a manner that its position can be determined by means of the position measuring unit. Alternatively, two reference probes may be provided, one reference probe being arranged on the body of the patient whereas the other is mounted on the patient table.
Before the medical intervention an image data acquisition unit, for example a magnetic resonance tomography unit, a computed tomography apparatus, an ultrasound device or an X-ray device has already formed an image data base in which there are taken up the individual 3D image data sets which have been acquired simultaneously with the periodic motion signal which is the same as that acquired during the intra-operative determination of the spatial positions, with each individual 3D image data set there being associated an individual motion phase within a period of the motion signal. Moreover, the reference probe must already be pre-operatively located at its ultimate point of application and its position relative to the 3D image data sets must be determined. This can be realized, for example, by constructing the reference probe in such a manner that it is also detected during the image data acquisition and that is recognizable in the individual 3D image data sets. Another possibility consists in determining the position of the reference probe relative to the image data acquisition unit, for example, by determining the position of the image data acquisition unit and the reference probe by means of the known position measuring unit.
On the basis of the motion phase during the determination of the spatial position of the medical instrument and the reference probe that 3D image data set which is associated with the same motion phase is then (intra-operatively) selected from the image data bank. The motion signal, or more exactly speaking the individual motion phase, thus quasi represents the link between the intra-operatively determined spatial position of the medical instrument and the pre-operatively determined 3D image data set which contains the information concerning the position of the anatomy in the same motion phase. Because the position of the reference probe relative to the body organ is also known in this 3D image data set and the actual spatial position of the reference probe was measured, a conversion formule can be determined therefrom; the measured spatial position of the medical instrument can thus be simply converted, for example by means of a simple co-ordinate transformation, into its position relative to the body organ. The invention thus enables exact determination of the position of a medical instrument, introduced into the body of a patient, relative to a periodically moving body organ, for example the position of a catheter introduced into a ventricle, and also the tracking of motions of the instruments. The physician can thus guide the instrument to accurately predetermined positions in which the desired interventions can be carried out.
Attractive versions and embodiments of the method according to the invention and the device according to the invention are disclosed in the further claims.
The decision as to which motion signal is recorded and used during the method according to the invention is dependent notably on the motion whereto the medical instrument introduced into the body is subject itself, or on the motion performed by the body organ in which or in the vicinity of which the medical instrument is to operate. In the case of a heart catheter this will notably be the cardiac motion, so that it is advantageous to record an electrocardiogram of the patient as the motion signal. In the case of interventions in the brain, an electrocardiogram is again suitable. In an advantageous embodiment a respiratory motion signal which is dependent on the respiratory motion of the patient is acquired as the motion signal.
In a further preferred embodiment such a respiratory motion signal is acquired in addition to another motion signal, for example the electrocardiogram, in order to be used during the determination of the position of the medical instrument in conformity with the described method so as to take into account and correct the motions of the anatomy, and possibly of the medical instrument, which are due to the respiration. This embodiment yields an even more accurate determination of position.
In a preferred embodiment, moreover, a 3D image data set is also acquired during the determination of the spatial position of the medical instrument and the reference probe; this can be realized by means of a real-time 3D ultrasound method and such a data set can be used during the determination of the position of the medical instrument relative to the body organ and/or can be taken up in the image data base. If desired, the accuracy of the position determination can thus be further enhanced. Moreover, information is thus made available concerning a real-time 3D image data set which can also be used for the formation of an image of the surroundings of the medical instrument from the selected 3D image data set, which image can then be displayed on a display device; the position of the medical instrument can be superposed on said image as is performed in a further preferred embodiment.